In practice, this subcarrier at 57 kHz is triple a reference frequency at 19 kHz, but it turns out that the observed precision in these frequencies is not optimal, in particular if the various emitters and the various receivers that are supposed to interact with one another are considered.
With this in mind, the receiver must have a mechanism for resetting carrier frequency, and phase if needs be, in order to be able to demodulate, under satisfactory conditions, the signals received via the antenna and to be able to deduce therefrom a so-called “baseband” signal that is exploitable with respect to so-called RDS information.
A feedback loop of the phase-locked-loop (PLL) type, developed in the historic context of purely analog radios is known to those skilled in the art.
This PLL solution has been adapted to the context of vehicle radios implementing digital processing, and is known as a “Costas loop”.
However, it turns out that the implementation of this “Costas loop” adaptation is not optimal from the economic point of view in the context of use of “finite impulse response” (FIR) digital filters and of vector digital signal processors (DSPs).